No currently known living or extinct bird has them. From ScienceDaily:
“In ostriches, the upper kneecap looks similar to the single kneecap in most other species, but the lower kneecap resembles a fixed bony process, like the point of your elbow,” says Ms Regnault. “As far as we know, this double kneecap is unique to ostriches, with no evidence found even in extinct giant birds.”
The effect that this double-kneecap has on the running performance of ostriches is hard to identify, but Ms Regnault and her team have a few ideas: “We speculate that this might mean ostriches are able to extend their knees relatively faster than they would with one kneecap.” Society for Experimental Biology More.
Ah yes, the usual evolutionary conundrum: It may be an advantage but that does not explain how the ostrich developed it.
See also: Big squawks over bird speciation?